1. Field of the Invention
The present invention relates to optical coherence tomography, and more specifically, it relates to the use of adaptive optics in optical coherence tomography for retinal imaging.
2. Description of Related Art
Optical coherence tomography (OCT) is a leading modality for non-invasive, in vivo imaging of the human retina, providing high sensitivity, speed and axial resolution in quantifying retinal pathology. The lateral resolution of the OCT image of the retina, however, is poor due to ocular aberrations.
Adaptive optics (AO) have been incorporated into OCT systems to increase the lateral resolution by measuring and subsequently compensating for the aberrations in real time. This technology was originally used for correcting image degradation due to atmospheric aberration in astronomy. In recent years, AO technology has been applied to several instruments for retinal imaging, such as flood illumination fundus imaging, scanning laser ophthalmoscopy, and most recently, ophthalmic optical coherent tomography. In an AO system, the ocular aberrations of the test subjects are measured by a wavefront sensor. The measured wavefront errors are then used to adjust the shape of a deformable mirror (DM) until the wavefront aberrations are minimized.
Population studies have shown that many people have both low-order aberrations with large magnitudes and high-order aberrations with small magnitudes. For these subjects, current technology cannot deliver the phase compensation needed using a single deformable mirror (DM). The use of two deformable mirrors has been investigated. An AO-OCT system incorporating two deformable mirrors was demonstrated by Zawadzki et al. The bimorph DM from AOptix used in that OCT system had a relatively high dynamic range and could correct defocus and astigmatism up to ±3D. This obviated the need for the meticulous use of trial lenses to correct the refractive errors of a subject. The system included a micro-electro-mechanical system (MEMS) DM (from Boston Micro Machine) that had 144 pixels and 1.5 μm stroke, which was used to correct the residual high-order aberrations left by the bimorph DM compensation. Both deformable mirrors were placed in the non-scanning path. Such arrangement, however, generated noticeable beam distortions at the deformable mirrors and the wavefront sensor when large refractive corrections were needed.
In an AO-OCT system demonstrated by Zhang et al., the bimorph mirror was placed one relay telescope away from the eye. This minimized the propagation of ocular refractive errors through the system prior to compensation. This arrangement greatly reduced the pupil distortion at the deformable mirrors and wavefront sensor. However, because the bimorph mirror was placed in the scanning path (i.e., between the eye and scanners), the beam at the eye pupil shifted with the changing incidence angles of the light as the beam was steered by the scanners. This would result in degradation of the AO-OCT system.